The present invention relates to centerless grinding, and more particularly, to a novel process and apparatus for minimizing the roughness error in the workpiece in such centerless grinding.
In centerless grinding the workpiece is not held at a fixed rotational axis, but is rotatably constrained about its periphery during grinding. Such a centerless workpiece setup can generally hold a tight part-to-part tolerance, provide a stiff support across the length of the workpiece, and be easily adapted to continuous through feed with zero loading time. Thus, centerless grinding has been viewed as one of the most efficient and rapid means of producing precision surface finishes on cylindrical components. The basic elements of a plunge centerless grinding system include a workpiece which is constrained by a grinding wheel, regulating wheel, and support blade. Moreover, the centerless grinding system is characterized mathematically by the machine setup including the cutting ratio K, and setup angles .alpha. and .beta. which are used to describe the contact positions between the workpiece and the grinding wheel, regulating wheel, and support blade.
One of the main difficulties in industrial applications of centerless grinding is that the quality of the ground workpiece is often adversely affected by the surface waves generated in the workpiece during grinding, causing the workpiece to be out-of-round. The problem of workpiece out-of-roundness in centerless grinding was recognized more than half a century ago. The cause of such a problem can be explained partially by the fact that the workpiece is supported about its periphery.
The tendency to generate workpiece roundness error results primarily from the geometry of the workpiece setup itself. Surface irregularities in the workpiece coming into contact with the regulating wheel or support blade will cause the workpiece center to move away from its original position. These movements of the workpiece center cause the grinding wheel to produce additional surface irregularities in the workpiece. The inherent tendency of regenerating the workpiece surface irregularities causes centerless grinding to be more susceptible to workpiece out-of-roundness than center-type grinding. In fact, workpiece out-of-roundness in centerless grinding is one of the main limitations of the centerless grinding process as a high rate and high precision process.
Choosing proper setup angles .alpha. and .beta. in order to control the workpiece roundness error has been of critical importance to the industry since the early years of centerless grinding. The state of the art in selecting the proper setup angles .alpha. and .beta. still largely relies on a machinist's trial and error procedure, and centerless grinding is often viewed as an unpredictable process or an art rather than a science. Most of the existing analyses address only one particular aspect of the workpiece out-of-roundness problem, and a general approach to the problem is not available.
Exemplary of a single variable approach is Unno et al U.S. Pat. No. 4,570,387 in which a conventional centerless grinding machine keeps the throat angle, .beta., at a constant, e.g., at 7 degrees. The constant throat angle is maintained by adjusting the workpiece center height to compensate the changes of the grinding and regulating wheel diameters, and workpiece diameter. A constant throat angle, however, does not assure improved workpiece roundness.
Another example of a single variable approach is Frost et al U.S. Pat. No. 4,926,603 in which a conventional centerless grinding machine detects the periodic irregularities on the periphery of the workpiece during grinding, and periodically modifies the workpiece center height by adjusting the height of the support blade to eliminate the surface irregularities. However, adjusting the blade height during grinding alone can not always effectively eliminate the irregularities and may introduce additional surface irregularities.
It is widely recognized that the setup angles .alpha. and .beta. play an important role in the workpiece rounding process, and a mathematical model, known as the rounding mechanism, has been developed to describe the geometric effect of the centerless workpiece setup.
Although progress in the field has been made by the formulation of the rounding mechanism, this mechanism is incomplete due to the fact that the surface wave generation process is influenced not only by the geometrical effect of the centerless workpiece setup, but also by the regenerative effect of the cutting process, the dynamic effect of the machine structure, and various types of grinding disturbances occurring during grinding. Thus, a system approach based on the causal behavior between grinding disturbances and the resulting workpiece roundness error is required to effectively address the workpiece out-of-roundness problem.
Accordingly, it is an object of the present invention to provide a novel system approach to minimize the workpiece roundness error in a centerless grinding system (lobing loop system) in which the resulting lobing response to the grinding disturbances is minimized.
It is also an object to provide such a method in which proper grinding and regulating wheels may be readily selected for the centerless grinding process based on the desired centerless grinding performance.
Yet another object is to provide such a method in which the workpiece roundness error is minimized by selecting the proper setup angles .alpha. and .beta. based upon the grinding disturbances occurring during grinding.
A further object is to provide such a method in which the workpiece roundness error is further minimized by regulating the ratio of the grinding wheel rotational speed to the workpiece rotational speed during the grinding operation.
Still another object is to provide a novel apparatus for practicing the novel method of controlling workpiece roundness error.